They said the four gunmen also vandalized vehicles in the area, including those of the U.S. Embassy, early Monday. The officials spoke on condition of anonymity because they were not authorized to talk to the media.

The U.S. Embassy, which is off Tahrir Square, said vandals attacked an Embassy van, slashing its tires and breaking a window. It warned U.S. citizens in a warden message against going to the square in downtown Cairo, where New Year's celebrations are planned later. The square was the center of Egypt's uprising two years ago.

The sit-in was to protest against Islamist President Mohammed Morsi's moves to pass a disputed constitution.

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